Inspired by this question: What makes education in Finland so good?
Finland has marketed itself as a top country in education. Indeed, at some time, the Pisa results in Finland were quite good. However, the Pisa results started going down when a significant number of immigrants arrived (this is based on recollection from studying to become a teacher in Finland; might be wrong, and please correct if it is), and the gap between their results and the results of the native population used to be big. This suggests that the high Pisa results might have been due to a homogenous population of native Finnish speakers, as much as due to a successful curriculum. But maybe not, and maybe Finland has a very good school system.
Is there a research-based reason to believe that the educational system in Finland is particularly excellent, or just average, or worse than average? Is there a consensus among researchers about this?